During knee implant surgery, more specifically known as knee arthroplasty, damaged or diseased bone is replaced with metal or plastic components to restore the function of the effected joint. Improvements in materials used in these implants have resulted in widespread acceptance of this surgical procedure.
A primary goal of knee arthroplasty is the proper placement of the components with respect to the anatomy of the patient. This placement is necessary so that proper implant function is achieved and the life of the implant optimized. In a human, the weight of the body passes along 8 theoretical line, sometimes referred to as the weight bearing axis or WBA, from the center of the hip joint to the center of the ankle joint. In a correctly functioning knee, the WBA passes through the center of the knee in both anterior/posterior planes and medial/lateral planes. In a knee exhibiting varus deformity the WBA passes medial to the center of the knee while in a knee exhibiting valgus deformity it passes lateral to the center of the knee. Location of the WBA during the knee arthroplasty therefore relies on the location of the center of the hip joint (the center of the head of the femur), the center of the knee, and the center of the ankle.
In addition to locating the WBA, the position of the implants along the WBA and their rotation around the WBA must be established. A horizontal line passing through the articular surface of the knee, referred to as the joint line, can be used to position the implants along the WBA. To rotationally locate the implants in the horizontal plane, several different anatomic landmarks are commonly used such as the location of he posterior or anterior femoral condyles or the location of the femoral epicondyles.
Most current techniques for locating the WBA can be grouped into one o two categories extramedullary or EM alignment; and, intramedullary of IM alignment.
EM alignment requires that the surgeon visually align slender, parallel rods from the knee joint to the head of the femur and the center of the ankle. The position of the head of the femur may be approximated either by palpation or with intraoperative X-ray. Location of the ankle rod can be approximated either with a notched device intended to seat around the ankle or with visual placement of the rod with respect to the palpated malleoli of the ankle. Once correctly positioned, the hip and ankle rods should lie parallel to the patient's WBA.
IM alignment replaces visually located rods with rods placed in the medullary canals of the femur and tibia. If properly placed, these rods should lie on the axis of the bones. Due to the offset of the femur at the hip, the bone axis is not the same as the WBA, therefore a correction must be made at operation to adjust the IM axis to estimate the WBA. This correction requires a preoperative X-ray be taken showing the angular difference between the femoral axis and the WBA.
EM alignment provides only visual estimation of the location of the WBA. It is subject to many errors and requires considerable surgeon-skill. Intraoperative location of the head of the femur is especially error prone. Palpation of the femoral head, complicated by patient obesity and sterile drapes placed over the patient, has been shown to be commonly inaccurate by 2-3 inches in comparison to radiographic location. Templates have been devised to assist radiographic location but the use of radiographs in the OR suite is time-consuming, awkward and exposes personnel to radiation. Additionally, any form of radiographic location is subject to distortion and requires a visual estimate of the location of anatomic landmarks which are not necessarily he kinematic centers of movement or of force transmittal.
IM alignment requires some skill in placing the rods. The placement of rods into the femur and tibia has been related to patient death from fat or gas embolism. The adjustment angle required o correct the IM axis to the WBA is commonly measured from a preoperative X-ray and, as result, is subject to distortion, reading errors and visual estimation problems as previously described.
What is required then, is a method for accurately and imply locating the true location of a patient's WBA determined by the kinematic position of the patient's joints. The method should be applicable to intraoperative applications. Preferably the method developed would not involve additional modification of the bone, other than that necessary for performing the procedure, could provide for preoperative planning and postoperative evaluation, and could be used to predict the effect of changes to the surgical protocol upon the results of the surgery.